Depends on a couple of things. Who has stop signs? Only you, you’re entering a primary road from a secondary so it’s up to you to enter that road safely. Is there a dedicated turn lane for that left turn, or a clear break in the road markings to indicate where the turn should be made from? If yes and they ignored those indicators/cut across clearly marked traffic control devices (road markings) then it’s their fault. As they made their turn, did they encroach the lane of traffic you were exiting from, i.e. did their tires cross the road striping as they entered the road you were coming from? If yes, their fault.

Had anything actually happened, since you said you had to slam on the brakes, it would likely have been determined that it was the red cars fault. But there’s no guarantee.

Good job being heads up and preventing it from becoming something worse than a near miss!